Eosin Y-catalyzed photooxidation of triarylphosphines under visible light irradiation and aerobic conditions

Article abstract of DOI:10.1039/c6ra25469a

We report herein a novel method for Eosin Y-catalyzed photooxidation of triarylphosphines under visible light irradiation and aerobic conditions. This new approach employed visible light as the energy source and air as the oxidant, showing great advantages in environmental benignness and operational easiness with a wide functional group tolerance.

Full text of DOI:10.1039/c6ra25469a

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Eosin Y-catalyzed photooxidation of
triarylphosphines under visible light irradiation and
Cite this: RSC Adv., 2017, 7, 13240
aerobic conditions†
b
b
b
b
a
b
*
*
Yanbin Zhang, Cong Ye, Shijie Li, Aishun Ding, Guangxin Gu and Hao Guo
Received 19th October 2016
Accepted 21st February 2017
We report herein a novel method for Eosin Y-catalyzed photooxidation of triarylphosphines under visible
light irradiation and aerobic conditions. This new approach employed visible light as the energy source
and air as the oxidant, showing great advantages in environmental benignness and operational easiness
with a wide functional group tolerance.
DOI: 10.1039/c6ra25469a
rsc.li/rsc-advances
Photochemistry is an important branch in modern chemistry.1 preparation of phosphine oxides from phosphines via direct
Distinct from traditional thermal reactions, the key intermedi- oxidations,14,15 among which some photocatalysts, such as 9,10-
ates in photo-reactions are the electronically excited state of dicyanoanthracen,15b 9-mesityl-10-methylacridinium perchlorate,15c
organic molecules. Photochemical sensitization of triplet oxygen and sensitizers generating singlet oxygen15d showed nice cataly-
to singlet oxygen is one of the appealing aspects of photochem- tic activity.15 However, in many cases, external environmentally
istry.2 Compared with chemical decomposition, electro-chemical, harmful oxidants,14a expensive additives,14b or harsh conditions14c
bio-chemical and other methods,3 photochemical sensitization is were required. Moreover, stoichiometric oxidants will generate
cheap, easy to handle, efficient and environment-friendly. More stoichiometric wastes, which makes the purication process much
importantly, organic photocatalysts are easy to functionalize, more complicated.14b An efficient, environmentally friendly, and
which means the catalyst could be attached to advanced mate- convenient method for the oxidation of phosphines is desirable as
rials4 and bio-materials.5 Several photocatalysts were used in part of an overall effort to develop “Green Chemistry”. With our
singlet oxygen generation.6 Among them, the structure of Eosin Y continuous interest in this eld,16 we turned to study the Eosin Y-
showed good features, such as aqueous solubility and easiness to catalyzed photo oxidation of triarylphosphines based on the
modify (Scheme 1). Thus, much attention has been paid to Eosin following considerations. Firstly, Eosin Y is cheap and the loading
Y-catalyzed singlet oxygen generation.7
of the catalyst is generally low. Secondly, harsh irradiation condi-
Phosphine oxide is an important fragment in medicine,8 and tions can be avoided, since Eosin Y absorbs visible light. Thirdly,
functional supramoleculars.9 It also plays an important role in Eosin Y is stable in air and absorbs visible light, thus, special
organic synthesis.10 It has been used as a catalyst for the Wittig glassware and protective gas is no more required. This will provide
reaction,11 a ligand for transitional metals,12 and as a gentle operational convenience. Here in, we wish to report our recent
oxidant.13 Numerous methods have been developed for the results in the photooxidation of triarylphosphines using Eosin Y as
the catalyst under visible light irradiation and aerobic conditions.
Our initial attempts were carried out using triphenylphos-
phine 1a as the model substrate, 15 mol% of Eosin Y as the
catalyst, a 23 W household lamp as the light source, and oxygen
as the oxidant. A quantitative yield of the corresponding triphe-
nylphosphine oxide 2a was obtained in all tested solvents (entries
1–6, Table 1). Given that the reaction in methanol showed the
Scheme 1 Structure feature of Eosin Y.
highest reaction speed (entry 6, Table 1), methanol was chosen as
the standard solvent. However, when we tried to explore the
substrate scope, some reactants were not dissolved in methanol.
Thus, a mixture of dichloromethane (DCM) : methanol (MeOH)
aDepartment of Materials Science, Fudan University, 220 Handan Road, Shanghai,
200433, P. R. China. E-mail: guangxingu@fudan.edu.cn; Fax: +86-21-65648837;
¼ 5 : 1 was used as the optimal solvent (entry 7, Table 1). Further
Tel: +86-21-65648837
studies showed that the loading of catalyst could be dramatically
bDepartment of Chemistry, Fudan University, 220 Handan Road, Shanghai, 200433, P.
reduced (entries 7–10, Table 1). We were delight to see that only 1
R. China. E-mail: Hao_Guo@fudan.edu.cn; Fax: +86-21-55664361; Tel: +86-21-
mol% of Eosin Y was enough to efficiently catalyze this trans-
55664361
formation (entry 10, Table 1). Air was then used as oxidant
† Electronic supplementary information (ESI) available: Experimental procedures,
and characterization data for all compounds. See DOI: 10.1039/c6ra25469a
13240 | RSC Adv., 2017, 7, 13240–13243
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Table 1 Optimization of the reaction conditionsa
Table 2 Photooxidation of 1 under condition Aa
Eosin
Y (mol%)
Time
(h)
NMR yieldb
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
THF
CCl4
DCM
Ethyl acetate
CH3CN
MeOH
DCM : MeOH ¼ 5 : 1
DCM : MeOH ¼ 5 : 1
DCM : MeOH ¼ 5 : 1
DCM : MeOH ¼ 5 : 1
DCM : MeOH ¼ 5 : 1
DCM : MeOH ¼ 5 : 1
DCM : MeOH ¼ 5 : 1
DCM : MeOH ¼ 5 : 1
DCM : MeOH ¼ 5 : 1
15
15
15
15
15
15
15
10
5
1
1
1
1
7
6
5
4
>99
>99
>99
>99
>99
>99
>99
>99
4
1.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
10
9
>99
>99
10
11c
12e
13g
14c
15c,h
>99 (95)d
91 (5)f
40 (55)f
30 (66)f
NR
—
1
a
A solution of 1a (0.2 mmol) in the tested solvent (3 mL in total) was
irradiated by a 23 W household lamp at rt under O2 atmosphere (1
b
atm). Yield determined by 31P NMR spectroscopy of the crude
reaction mixture using tricyclohexylphosphin as internal standard.
c
The reactions were carried out under air atmosphere (1 atm).
d
f
e
Isolated yield. Eosin Y disodium salt was used instead of Eosin Y.
g
Recovered yield of 1a. Fluorescein was used instead of Eosin Y.
h
The reaction was carried out without light.
instead of pure oxygen, the same excellent yield was observed
(entry 11, Table 1) with high quantum yield (F ¼ 0.72). Eosin Y
disodium salt was then used instead of Eosin Y but showed
slightly decreased yield (entry 12, Table 1). Non-halogenated
Eosin Y derivative, uorescein, was also tested but showed far
more less reactivity (entry 13, Table 1). Finally, control experi-
ments without Eosin Y (entry 12, Table 1) or light (entry 13, Table
1) were carried out. The results indicated that both Eosin Y and
light played a key role in this transformation. Thus, condition A
(Eosin Y (1 mol%), DCM : MeOH ¼ 5 : 1, air (1 atm), 23 W
household lamp, and rt) was applied for further studies.
With the optimal conditions in hand, we next examined the
substrate scope of this photooxidation with a series of triar-
ylphosphines (Table 2). Reactants with a strong electron donating
group, like methoxy (2b) or ethoxy (2c), were rstly examined,
which gave the desired products in excellent yields. Then some
weak electron donating groups, such as methyl (2d, 2e, and 2f),
ethyl (2g), tert-butyl (2h), and phenyl (2i), were induced into the
aryl ring of the triarylphosphines and their reactivities were
tested. Good to excellent yields were generated. Electron decient
substrates with weak electron withdrawing groups (2j and 2k) or
strong electron withdrawing groups (2l, 2m, and 2n) were then
applied under the standard conditions. The corresponding yields
were quite satisfying. Notably, trinaphthylphosphine (1o) and
triphenylphosphine (1p) were also tolerant and showed nice
reactivities in this reaction. These results demonstrated that
a
A solution of 1 (0.2 mmol), Eosin Y (1 mol%), DCM (2.5 mL), and
MeOH (0.5 mL) was irradiated by a 23 W household lamp at rt under
air atmosphere (1 atm). Isolated yield was reported.
tolerance for a wide range of arylphosphines was achieved in this
photooxidation methodology.
To gain insight to the mechanism, singlet oxygen quenching
experiments were carried out using 9,10-dimethylanthracene as
singlet oxygen quencher.2b The results were shown in Fig. 1. The
whole reaction process was inhibited by 1 equivalent of 9,10-
dimethylanthracene (triangle symbol). The reaction rate was even
lower with the addition of 2 equivalents of 9,10-dimethylan-
thracene (square symbol). Since 9,10-dimethylanthracene was
a typical singlet oxygen quencher,2b we reasoned that singlet
oxygen might be the key oxidative species in the triarylphosphine
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triarylphosphadioxane 3.18a 3 and another molecule of 1 nally
furnished 2 as the product.18b
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In conclusion, a visible light-induced metal-free photo oxida-
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as the catalyst and air was used as the oxidant. Considering that
this protocol is cheap, environmental friendly, easy to operate,
and the substrate scope is broad, this method may be widely
used in the oxidation of phosphine derivatives.
Acknowledgements
We greatly acknowledge the nancial support from Interna-
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(2014DFE40130).
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